BSlatt,
Is it possible to post a photo of your terrarium jug? I agree with WillC, unless one is an expert at the artistry of terrarium keeping, those types with the very narrow openings are quite impractical and difficult to keep healthy.
The possible reason for the mold and death of the plants contained in your terrarium may have been due to the fact that the "jar" has a small opening, not allowing proper air circulation; if the cap remained in place for extended periods, it probably caused the mold spores to flourish. Plants need air, nutrients and water to survive and if one of those elements is absent, plants won't live very long.
Depending on the height of the jar, you will need long tongs to remove old plant material and add new plants if you make the decision to do so. I'd suggest leaving the cap off the terrarium to allow the soil to dry thoroughly before adding new plants.
